This invention relates generally to the monitoring and/or controlling of the ratio of hydrogen to water vapor in metal heat treating furnaces.
In heat treating or thermal processing of metal and metal alloys, metal parts are exposed to specially formulated atmospheres in a heated furnace. Usually, the atmosphere contains the gaseous species hydrogen H2(g) and water vapor H2O(g). For example, the atmosphere can comprise a mixture of nitrogen N2, hydrogen H2, and water vapor (steam) H2O. Alternatively, the atmosphere can comprise an exothermic-based atmosphere, generated by an external exothermic generator to contain a mixture of carbon monoxide CO, carbon dioxide CO2, nitrogen N2, hydrogen H2, and water vapor H2O.
The hydrogen to water vapor ratio in these atmospheres (in shorthand, called the H2/H2O ratio) can affect the metal parts being processed and therefore should be monitored. The magnitude of the H2/H2O ratio at a given temperature relates to the presence or absence of oxidation. More particularly, based upon thermodynamic considerations, oxidation of metal parts at a given temperature occurs when the H2/H2O ratio of the atmosphere is lower than the H2/H2O ratio at which equilibrium of the metal to its oxide at that temperature exists, which in shorthand will be called the equilibrium ratio.
The equilibrium ratio for a given metal at a given temperature for a given type of atmosphere can be approximated using, e.g., an Ellingham diagram (see Gaskell, Introduction of Metallurgical Thermodynamics, p. 287 (McGraw-Hill, 1981). The actual H2/H2O ratio of the furnace atmosphere is usually determined by using remote gas analyzers. Remote gas analyzers individually measure percent hydrogen content and the dew point of the atmosphere, which is a measure of the water content. From these two measured quantities, the H2/H2O ratio of the sampled furnace atmosphere can be ascertained by conventional methods.
Remote sensing of percent hydrogen content is accomplished using conventional thermal conductivity analyzers. These analyzers are generally well suited for sensing H2 content in simple, binary gas atmospheres, containing a mixture of H2 and N2 gases. However, conventional thermal conductivity analyzers are not as well suited to sense H2 content in more complex exothermic-based atmospheres, where carbon monoxide and carbon dioxide are also present with nitrogen.
In addition, the process of remote gas sensing can itself create significant sampling errors, which lead to erroneous readings. Remote gas sampling requires withdrawing atmosphere gas samples out of the furnace through gas sampling lines. The analysis is performed at ambient temperatures, and not at the temperature present in the furnace, so the sample must be cooled. These physical requirements for remote analysis introduce sampling errors, which are difficult to eliminate.
For example, error may arise due to leaks in the gas sampling line. Another error may also arise due to alteration of the gas chemistry caused either by soot formation during cooling (which is governed by the reaction: CO+H2=C+H2O), or by a water gas shift in the atmosphere (which is governed by the reaction: H2O+COxe2x86x92CO2+H2), both of which alterations are a function of the sampling flow rate. Furthermore, in the case of high dew point atmospheres, condensation of water in the gas sampling lines can occur, leading to erroneous sensing results. All or some of these errors can occur at the same time.
The dew point of an exothermic-based atmosphere is usually measured when the atmosphere is produced by a separate external generator. However, this measured dew point does not relate to the dew point of the atmosphere once it enters the heated environment of the furnace itself. This is because, exothermic-based atmospheres are cooled to reduce their water content before introduction into a heated furnace environment. The cooling leaves the atmosphere in a non-equilibrium condition in reference to carbon dioxide CO2 and water H2O. When reheated to thermal processing temperatures inside the furnace, these gases react to reach equilibrium, generating water to prescribe a new dew point and percent carbon dioxide content, according to the reaction: CO2+H2=CO+H2O.
For these reasons, there is a need for more direct and accurate systems and methods to ascertain the actual H2/H2O ratio in atmospheres during the thermal processing of metals and metal alloys. There is also a need for systems and methods to apply the ascertained H2/H2O ratio for control and for record keeping purposes.
One aspect of the invention provides systems and methods for monitoring a metal heat treating atmosphere by generating a computed H2/H2O ratio for the atmosphere as a function of temperature and oxygen partial pressure PO2.
In a preferred embodiment, the PO2 is sensed in situ by a zirconia oxygen sensor. The temperature is likewise sensed by an in situ thermocouple. The in situ oxygen sensor and thermocouple are installed in the thermal processing furnace in direct contact with the gas atmosphere. This obviates sampling errors that are inherent in remote gas sampling techniques.
Another aspect of the invention provides systems and methods that make beneficial use of the computed H2/H2O ratio. For example, the systems and methods control the thermal processing atmosphere based, at least in part, upon the computed H2/H2O ratio, e.g., by controlling the mixture of gases in the atmosphere. As another example, the systems and methods record or display the computed H2/H2O ratio, or both.
Another aspect of the invention provides systems and methods for monitoring a metal heat treating atmosphere by deriving from at least one sensor placed in situ in the atmosphere a process variable indicative of the H2/H2O ratio. The systems and methods make use of the process variable, e.g., by displaying the computed H2/H2O ratio, recording the H2/H2O ratio, or by using the H2/H2O ratio as a process variable to control the atmosphere.
Other features and advantages of the inventions are set forth in the following Description and Drawings, as well as in the appended Claims.